This invention relates to cleaning of calcium sulfate scale from wells and particularly to cleaning by conversion of the calcium sulfate scale to an acid soluble form.
When the producing rate of an oil or gas well, for example, declines, a workover job is generally performed to increase production. The workover may include bailing out of accumulated scale and other solids, acidizing, fracturing, use of paraffin solvents, or scale solvents, and the like. Removal of calcium sulfate scale from wells has presented problems, particularly when the scale is in a massive, dense, impermeable form, or when the scale is deposited in pores or other openings within the formation.
In some instances, direct solvents for calcium sulfate (or gypsum) have been used. In U.S. Pat. No. 2,396,938, Bersworth suggests the use of ethylene diamine tetracetic acid, but its use in oil wells has been limited by its cost and relatively low effectiveness. In hydrocarbon producing wells, the chemicals are usually much less expensive ones such as hydroxy acetic acid (glycolic acid), as mentioned in U.S. Pat. No. 3,547,194, by Morine, or simply sodium hydroxide.
In U.S. Pat. No. 3,688,829, I described a direct solvent for calcium sulfate scale containing generally the same gluconates and hydroxides as described herein, and in somewhat similar proportions, but without any carbonate as described herein. The direct solvent solution of my prior patent has been sold under the trademark "C-Dex".
In other prior art techniques, converters have been used. In U.S. Pat. No. 2,787,326, Hughes discloses several converter solutions, including sodium and potassium carbonates (it is to be noted, however, that wellbore deposits of these carbonates do not occur naturally). After such a conversion reaction is completed, Hughes' water-insoluble reaction product (calcium carbonate) is treated according to known acidizing techniques (a 10-30% acid solution is introduced into the wellbore, following which the well is flushed to remove residual acid, reaction products, and particles of scale loosened by the reactions).
Even in the case where direct solvent solutions are used, a follow-up acid treatment is often performed to remove scale of types other than calcium sulfate, especially calcium carbonate and iron sulfide scales.
In the past, either the direct solvent or the converter techniques have generally required extensive soaking times, and while occasionally a well with little scale might be treated in as little as four hours, it has become more or less standard practice to maintain the solvent or converter in the wellbore for about 24 hours. As lost production costs can often be $10,000 or more per day, the cost of such 24-hour soaking times can be quite significant.